This is an indication that the measure numbers are discontinuous. The numbering goes from 1 directly to 6, so presumably there were measures 2-5 that got cut at some point.
This happens frequently in musical theater, because during the original production, the music is constantly being adjusted and measures are added or removed. However, the entire production has already started to refer to points in time by measure number, so you can't change measure numbers after they've been written. For example, the Lighting Designer has already decided that a particular lighting cue will happen at m. 50, and if you change the measure numbers, then everyone's notes go out of date and it's pure chaos. So instead, we deal with discontinuities like this.
When measures are added, they'll get a letter suffix. So if it's decided that an intro isn't long enough and 4 more measures need to get added between m. 4 and 5, then those measures get numbered 4A-4D.